In a spark plug that is used in an internal combustion engine such as an automobile engine, an alumina material sintered body made by sintering an alumina (Al2O3) insulating material is practically used as a spark plug insulator (hereinafter also referred to as simply “insulator”) which constitutes the spark plug. The reason for using the alumina material sintered body as the insulator is that alumina is excellent in heat resistance, mechanical properties and dielectric strength. Generally, the insulator is formed into an elongated shape in an axial direction thereof and formed with a through hole that extends in the axial direction of the insulator. The through hole receives a center electrode that is opposed to a ground electrode so as to form a spark discharge gap therebetween, and a terminal electrode for applying high discharge voltage to the center electrode.
The insulator is produced by baking a compact formed of raw material powder. More specifically explaining a generally known method for manufacturing the insulator, first, raw material powder containing alumina powder as a main component is mixed with an organic bonding agent (i.e., a so-called binder) and water as a solvent. The thus formed admixture is wet-blended to form a slurry. The slurry is dried by a suitable method such as spray drying, and then granulated. Next, the granulated material is formed into a compact having a given shape of the above-described insulator and then baked to thereby produce the insulator. There has been conventionally used the raw material powder containing alumina powder as a main component as described above, and specifically, the raw material powder having an average grain size of 2 μm in view of stabilization of quality of the insulator and cost saving and productivity of the insulator (for instance, see Patent Literature 1).
The insulator formed by the baking was shrunk by about 20% as compared to that before being baked. In a case where the shrinkage occurs non-uniformly upon the baking, bending (warpage or deflection) occurs in the insulator. The bending of the insulator causes a tip end of the center electrode to be displaced from an axis of the center electrode upon assembling the center electrode to the insulator. If the tip end of the center electrode is displaced from the axis of the center electrode, there may occur ignition failure that in turn causes failure in ignition which tends to be ordinarily generated between the center electrode and the ground electrode. That is, there may occur a so-called transverse flying spark that is generated between the center electrode and a main metallic fitting that is disposed on an outer circumferential periphery of the center electrode. Therefore, the bending of the insulator is one of most considerable defects which lead to deterioration in performance of the spark plug.
Conventionally, there have been proposed a method for inspecting insulators for bending in which the insulators are subjected to visual inspection as to whether or not any deflection is caused in the insulators while the insulators are rotated one by one about a longitudinal axis thereof.
However, in the above-described inspection method for the insulator, since the presence of the bending of the insulator is determined by the visual inspection, excessive detection of the bending will be frequently caused to thereby decrease the inspection accuracy.
Patent Literature 1: Japanese Patent Application First Publication No. 2002-305069